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PHOTO BY ISAAC NASAR
A precautionary shutdown is the most common reason for a single-engine landing.
hands, the spring tension holding the prop in the feathered position is significant – brute hand strength is insufficient. I always believed that a device known as a “prop-paddle” was an acceptable un-feather tool. Hartzell told me that it is not and internal damage is possible if attempted. And this new prop-paddle knowledge is why I stopped using a homemade version of the tool, stopped flying the Duke and awaited the annual inspection for diagnosis and repair. It took calls to the Tiffin Aire prop shop, Duke guru Bob Hoffman and Hartzell Propeller’s technical help-line to explain to me the intricacies of propeller mechanics so as to develop a plan of action. Eventually, Lycoming Service
Instruction 1462A provided the answer.
Full Feathering Props 101
Piston engine, controllable-pitch propellers come in several flavors, but the majority share a common design. Prop blade angle is controlled by the motion of a piston inside the propeller dome. This piston is moved by oil pressure on one side, with counterweights and a strong spring (usually augmented by a charge of compressed nitrogen in the prop dome) on the other. The counterweights do most of the work to drive the blades into feather (high pitch). The springs help to feather the prop when the centrifugal loads acting on the counterweights do not have enough centrifugal force to drive the blades completely to feather. As oil pressure changes, the piston moves, and the blade angle changes through gearing between the piston and the blades themselves. If oil pressure drops below a set minimum, the propeller blades drive to the feather position. In this situation with most twins, the propeller goes to so high a pitch that the blades flatten out relative to their direction of rotation, being twisted to the low-drag, feather position (82 degrees in the Duke, for example).
To drive the blade angles from feather (high pitch low RPM) back to low pitch high RPM, or to keep them there, you must have oil pressure. The oil psi of which we speak is not only from the engine’s oil pump (the psi you read on the gauge), but the prop governor gear pump boosts oil pressure before it heads out of the governor and into the propeller hub. With the boosted pressure, you get a better, quicker response
from the propeller when you move the lever in the cockpit. Low oil pressure can occur intentionally through the use of this blue propeller control lever by pulling the handle to the feather position; this opens a valve that dumps all oil from the prop dome and drives the blades into feather. Or, the low pressure can occur due to a massive oil leak or a “mechanical failure.” The mechanical failure mode is what I was experiencing with the right prop in the Duke – more on that and Lycoming SI 1462A in a bit.
Feather Me Not
Under normal operation, a component called an “anti- feather lock” is supposed to catch the prop as it attempts to feather when RPM and oil pressure decrease. The anti- feathering lock pins are held out of contact by flyweights when the engine is running and engage when propeller speed drops to 600 to 800 RPM. But for the anti-feather lock to move into place, there still must be a certain amount of oil pressure as the engine slows to a stop. Conversely, while sitting on the ground with the engine shut down, the prop sits on that latch until you start the motor. The locks help keep some oil in the prop dome and avoid friction and vibration when we shut down the engines.
In an in-flight emergency or training scenario with air load driving the propeller blades, moving the prop control to feather causes the prop blades to twist to feather pitch before the lock pins engage, and the propeller feathers. During a normal on-ground shutdown, the air load is absent, and when the engine shuts down, the prop reduces RPM slowly enough that the anti-lock pins drop into place as the rpm drops through that 600 to 800 RPM range. The prop blades will twist no further, so they don’t go into feather. No
A professionally made prop-paddle by Tiffin Aire.
November 2021 / TWIN & TURBINE • 25